Feldspar evolution in the Roxby Downs Granite, host to Fe-oxide Cu-Au-(U) mineralisation at Olympic Dam, South Australia

Abstract

The textural relationships and geochemistry of feldspars from least-altered to sericite-hematite altered and mineralised ~1.595Ga Roxby Downs Granite (RDG) at Olympic Dam, South Australia, were examined. The sample suite is representative of RDG both distal (>5km) and proximal (<1km) to the hydrothermal breccias of the Olympic Dam Breccia Complex (ODBC), which host Fe-oxide Cu-Au-(U) mineralisation at Olympic Dam. Microscopic observations and quantitative analyses indicate that a range of feldspar reactions have taken place within the RDG hosting the Olympic Dam deposit. An early phase of igneous plagioclase (~An27–34) is recognised, along with a more abundant, less-calcic plagioclase (~An12–20) both displaying rapakivi and anti-rapakivi textures with alkali feldspar. Alkali feldspars (~Or55Ab43An2) record post-magmatic evolution from cryptoperthite to patch perthite. Subsequent patch perthite is overprinted by highly porous, near end-member albite and K-feldspar, while plagioclase undergoes replacement by albite+sericite±Ba-rich K-feldspar. In sericite-hematite altered and mineralised RDG along the margin of the ODBC, sericite replaces all plagioclase, whereas red-stained, Fe-rich K-feldspar persists. Sulphide-uranium-rare earth element mineralisation is observed in association with hydrothermal feldspars, and increases in abundance with proximity to the orebody. Petrographic observations and whole-rock geochemistry illustrate the transformation of plagioclase and alkali feldspar from igneous to hydrothermal processes, and indicate that hydrothermal albite and K-feldspar formed within the RDG without the need for an external source of alkalis. Feldspar geothermometry indicates a minimum crystallisation temperature of 765°C at 2.2kbar for alkali feldspar (pressure estimate obtained using plagioclase-amphibole geobarometry) followed by a range of lower temperature transformations. Late-stage magma mixing/contamination is postulated from supportive temperature and pressure estimates along with feldspar and mafic mineral relationships.